I searched Amazon and Party City's websites. To my surprise Party City was actually less expensive for plates, cups and the wall decorations.
Alex wanted cupcakes with candy Lego's on top. His Grandma had given him candy legos before so I knew they existed. I found them on Amazon:
Candy Lego Bricks
They were the hit of the party!
For one of the activities I just filled a few bins with assorted legos and gave each kid a flat lego base and they just got to build for about 30 mins. The girls got bored before that time was up but the boys loved it!
Another easy thing was to fill a jar or vase with Legos and make a centerpiece. With older kids you could even have them all guess the number of legos in the jar and the winner gets to keep them. But with 4 & 5 years olds it was just a cool centerpiece.
It was a great party. Alex and his friends had a great time.